The invention relates to a line deflection circuit for an image display tube with magnetic beam deflection, provided with a series arrangement including a deflection inductance, a first and a second S capacitor, switching means being conductive during a part of the line period called the trace time for applying to the series arrangement a substantially constant voltage and being cut off during the rest of the period called the retrace time during which the series arrangement forms, with a retrace capacitance, a resonant circuit, the circuit being provided with modulating means for causing to flow in the second S capacitor a sawtooth current at line frequency and of amplitude varying at field frequency, a portion of this current also flowing through the deflection inductance.
Because the angle between the deflected cathode beam and the screen of the tube varies during the scanning, it is necessary to correct the scanning speed as a function of the coordinates of the scanned point: this is the wellknown correction called "S correction" because it gives an "S" shape to the representation of the sawtooth current in the deflection inductance (the deflector). It is obtained by means of the abovementioned S capacitor which is in series with the deflector during the trace time, during which time a constant voltage is applied to the series deflector-capacitor group called here the LC group.
In this way, a tuned circuit has been produced introducing a sinusoidal component which distorts the main current and gives it an "S" shape.
The basic circuit als requires a certain number of corrections intended to correct the geometric imperfections of the cathode ray tube/deflector assembly. In particular, a known distortion called "pincushion" distortion requires a correction known as "East-West" which causes a dynamic horizontal amplitude variation during the field scanning.
Numerous circuits have been proposed for correcting pincushion distortion. The circuit most used at present is that called the "diode modulator", which is described among others in the French Pat. No. 2,216,722, and more recently in the document: "Philips electronic components and materials"--Technical publication number 201, entitled "Drive circuits for 45 AX". The problem solved by this circuit is to be able to dynamically vary the amplitude of the line deflection without varying the extrahigh tension ("EHT") which is derived, in most cases, from the line deflection stage. In this circuit, a second LC group is disposed in series with the first one and a variable voltage at field frequency is applied to the terminals of the capacitor of this second group. This circuit requires adaptations with the new flat, squarecornered tubes; in fact, these tubes demand a more pronounded "S" correction than the old tubes, and more dynamic, i.e. varying during a field, whereas they demand less pincushion correction than the old tubes. In the circuits of the quoted documents, the application of the variable voltage at field frequency essentially causes a variation in the deflection amplitude. A variant, described for example with FIGS. 3 to 5 of the last document quoted, enables the additional introduction of a dynamic "S" correction (also called: "inner pincushion distortion") associated with the amplitude correction.
The obtaining of a smaller amplitude variation is now required, associated with a greater variation in the "S" correction. A known means for obtaining this effect is to add a second "S" capacitor in parallel with the first and to switch this second capacitor by means of an electronic switching circuit which connects or disconnects this second capacitor according to a duty cycle which varies during a field. This second capacitor has little effect on the amplitude. Such a circuit is described in document EP-A-1,046,345, according to which a capacitor thus switched is added to a conventional diode modulator circuit.
This technique has the disadvantage of requiring an additional circuit for defining the times of switching the additional capacitor, whose connection time must also be rather finely chopped in order to avoid the introduction of distortions in a line: switching it just once during a line is not sufficient as this would introduce a visible distortion in the line linearity.